On 2/29/12 1:12 AM, Lukas Czerner wrote: > On Tue, 28 Feb 2012, Eric Sandeen wrote: > >> I've been testing Lukas' last 2 patches for e2fsck discard, and noticed something a little odd. >> >> If I make a 512M file, loopback mount it, and mkfs.ext4 it with discard, it uses about 17M at that point. >> If I then run fsstress on it with a known seed, then run e2fsck -E discard on it, it uses about 52M. >> >> If I repeat the above test telling mkfs.ext4 NOT to discard, I'm left with about 94M after the discarding e2fsck. >> >> So it seems that perhaps e2fsck is not discarding everything that it could; after a discarding fsck, we should be left with the same (minimal) nr. of blocks "in use" no? > > The reason is (as I commented in the patch #2) that we will not discard > BLOCK_UNINIT groups. We use BLOCK_UNINIT as a optimization measure to > skip groups which are likely to be non-provisioned, because we have > never written there anything since the mkfs. > > If you create file system without discard, then obviously nothing is > discarded, image is fully provisioned and e2fsck discard *only* initialized > groups. So you'll end up with the bigger image, in case that your image was > not sparse. > > I hope that makes sense. It does, sorry, I had been focusing too much on patch #1 ;) -Eric > Actually I want to make the same optimization for fitrim. We discussed > it with Ted and Phillip (see the discussion under [RESEND] [PATCH 2/2 > v2] e2fsck: Do not forget to discard last block group. They did seem to > be convinced by that, however I think it is right thing to do for the > reasons I gave in that thread. > > Thanks! > -Lukas > >> >> I guess that's better than discarding _more_ than it should though. ;) >> >> (I suppose it is possible that this is the underlying filesytem being selective about which discards it accepts, but it behaves the same way on ext4 and xfs backing filesystems) >> >> -Eric >> >> FWIW, sequence of events here, tested with and without "-K" on mkfs.ext4: >> >> dd if=/dev/zero of=fsfile bs=1M count=512 >> losetup /dev/loop0 fsfile >> mkfs.ext4 -F /dev/loop0&>/dev/null >> mount /dev/loop0 mnt/ >> /root/git/xfstests/ltp/fsstress -s 1 -d mnt/ -n 2000 -p 4 >> umount mnt/ >> e2fsck/e2fsck.static -fy -E discard /dev/loop0> fsck1.out || exit >> du -hc fsfile >> losetup -d /dev/loop0 >> >> > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
